Refractory and abrasive material and process of making same



' Patented Apr. 7, 1942 REFRACTORY AND ABRASIVE MATERIAL AND PROCESS OFMAKING SAME Raymond c. Benner, Henry N. Baumann, Jr.,

and George J. Easter, Niagara Falls, N. Y., as-

signors to The Carborundum Company,

Niagara Falls, N. Y., a corporation of Delaware No Drawing. ApplicationMarch 21, 1939, Serial No. 263,294

11 Claims. (Cl. 106-58) The present invention is a continuation-in-partof copending application Serial No. 109,544 filed November 6, 1936,which in turn was a continuation-in-part of a previous applicationSerial No. 103,302 filed December 20, 1933. It relates to fusedaluminous compositions modified by interfusion therewith of chromite orewhereby a solid solution of chromic oxide in alumina is produced in theform of trigonal crystals. A particular feature of the invention is theprevention of the formation of an objectionable amount of crystallinespinel in the final body as a result of the presence in the fusion ofthe magnesia and iron which are introduced in the chromite ore, since wehave found that the presence of too much such spinel adversely affectsthe properties of the final body both as a refractory and as anabrasive.

It has been known for some years that pure alumina and pure chromicoxide form a continuous series of solid solutions together and while theadmixture of chromic oxide with alumina was vaguely suggested many yearsago as a likely refractory material, the cost of chromic oxide in even"commercia pure form has been so high that no practical development hasoccurred in that line. It occurred to the present applicants thatfusions of alumina with chromite warranted investigation however with aview to producing refractories of the trigonal crystalline solidsolutions at a cost which would be comparable with that of other widelyused special refractories, and the present invention has been the resultof their research in that direction.

While it appears that some beneflciation of the alumina occurs from arefractory standpoint as a result of the addition thereto of even smallamounts of chromite, we have found that a refractory of outstandingmerit is produced if the total percentage of alumina plus chromic oxideis kept above 80% and the chromic oxide constitutes at least 8% of thetotal weight. The highest possible proportion of chromic oxide isdesirable and it is in many cases possible to increase it to or or evenby judicious selection of the chromite ore and by furnacing in such away as to reduce out part of the iron while still leaving the chromiumin oxide form. In such cases the percentage of alumina decreasescorrespondingly from 70 or 80 to as low as In order however that thedesired trigonal crystal form may result it is necessary not only thatthe alumina and chromic oxide have the values stated but that the otheringredients have rather definite relations to one another. The

percentage of magnesia is particularly critical, mo

and while that of iron oxide is less so, iron oxide also acts in thesame way as the magnesia. A reason for this will perhaps be apparentfrom a consideration of the molecular weights of the various oxideswhich are present and the fact that spinels have a strong tendency tocrystallize from molten magmas containing these oxides.

The molecular weights are:

MgO 40 A1203 102 FeO '72 ClzOa 152 total weight. In this spineladditional alumina' goes into solid solution as pointed out by Easterand McMullen in Patent No. 2,063,154 so that the aggregate of apparentspinel crystals readily grows to predominant proportions. Whil suchspinel is for many purposes an excellent refractory, it is as previouslynoted not as good for some purposes (for example resistance to basicopen hearth steel slag) as is the trigonal chromealumina refractorywhich is the subject of the present invention. Also when used forabrasive purposes the spinel is definitely softer than the corundum typecrystal and hence is not desirable.

While it is not always necessary that the presence of spinel becompletely prevented it is for many purposes advantageous that it bekept down to not over 20% or even less of the volume of the refractoryand that the remainder of the refractory be composed of the trigonalcrystals described above. Our problem thus became the discovery of meanswhereby the formation of spinel crystals could be materially if notcompletely inhibited orthe determination of com-' position limits withinwhich it did not occur. The composition of chromite varies between widelimits. the following being perhaps common ranges:

Per cent CrzOs 40 to 58 1mm 7 to 25 FeO 12 to 25 Feioa o to 10 5 to 15In introducing 8 to 30% CraOa into a fusion, there is therefore anattendant introduction into the melt of 3 to 12% FeO and 1 to 7% MgO.Research showed that so long as the MgO and the FeO did notsimultaneously exceed 2 and 5% respectively, there was no serious spinelformation. The FeO can be maintained at about this maximum in the finalproduct by reduction of the iron oxide to metal in the furnace but theMgO is diflicult to remove without disturbing the CrzOa content also,hence must be otherwise cared for if spinel formation is to be avoidedin cases where ores high in magnesia are used.

We find that the introduction of small amounts of acid oxides such asSiOe, TiOz, ZX'O: or 1320: into the melt materially reduces the tendencyof the additional MgO to cause spinel formation.

Minimum Minimum Maximum SiO; TiO: MgO

This requirement may be met by silica and titania introduced with thealumina or silica may be added to the melt directly. A small excess ofthe acid oxide does no harm so that in general 3V6 have 2 t 7% of acidoxides in our final prod- 'uct. If however the magnesia runs high caremust be taken that the acid is also proportionately high if theproportion of spinel is to be kept below 20%. Chromites which areinherently low in magnesia are occasionally found. When it is possibleto selectsuch ores we prefer to do so, and to so proportion theingredients of our fusion' as to yield not over 2% MgO in the finalproduct. In such cases as in others it is desirable to reduce the FeO toaround 5 or 6%. While the effect of impurities is somewhat complicated,the basic principle is that the magnesia should be counteracted by otheroxides which have a strong tendency to combine with it and hence to tieit up in such a manner as to decrease the tendency to form spinels withchromic oxide and alumina.

In the fusions resulting in a trigonal crystal product, it appearsprobable that the iron oxide goes into solid solution to some extent inthe crystalline phase and that the remainder com-.

bines with the impurities in the interstitial areas. Titania is alsoknown to form solid solutions in alumina crystals and it is possiblethat some of the titania in these fusions is thus taken up, henceaccounting for the fact that titania is slightly less effective thansilica per unit of weight in counteracting the effects of magnesia,although the higher molecular weight of the titania would also tend inthis same direction. The magnesia together with silica. etc., seems toin general form an amorphous intercrystalline matrix between thetrigonal crystals rather than to form crystalline magnesium silicates,although in bodies very high in magnesia and silica a very few suchcrystals are sometimes discernible.

In carrying out this invention in accordance with one of our preferredmethods, we use a good grade of aluminous material containing at least95% A1203, the impurities being chiefly silica and oxides of iron andtitanium. We have also used alumina as high as 99% in A1203. The CrzO:content is obtained by adding selected commercial chromite ores high inCrzOa content. A little chromic oxide may also be added.

As an example of the use of a commercially available ore, and the resultof its treatment by our process we may take the following tabulationsshowing the composition of the original chromite ore and of the materialresulting from the fusion of this ore with three times its own weight'offused alumina scrap grain.

Composition of ore Composition 01 product The increase of silica andtitania is accounted for by their presence in the impure alumina scrapused. The product consisted of over per cent of well intergrownchrome-corundum crystals with the residual oxides entirely inclosed asisotropic interstitial material. If pure alumina had been used theamount of chrome-corundum would have been smaller and instead of thetypical trigonal form of crystallization, a considerable proportion ofchromite-spinel with alumina in solid solution would have been obtained.

In carrying out our invention we have found it satisfactory to use anelectric furnace similar to that commonly used for the production offused alumina for abrasive purposes, generally consisting of awater-cooled ironshell without any lining other than that built up bythe material being fused as it is fed into the furnace. Fusion iseffected initially by the heat from an electric are between 2 or moreelectrodes inserted in the iron shell; but after a bath of moltenmaterial is formed, the resistance of this material to the passage ofelectric current through it is used to supply heat. The material isgradually fed in and the electrodes raised as the fused mass is builtup. Apparatus of this type is shown and described in the patent to F. J.Tone, No. 929,517, granted July 27, 1909.

Economies may, under some circumstances, be effected by fusing an orewhich is a source of one of the oxides in the electric furnace, and thenadding the source of the other oxide to the molten contents of thefurnace. For instance, we may fuse bauxite, following the procedure bestsuited to eliminate impurities as is well known in the manufacture offused alumina abrasives, and then add the chromite ore to the moltenmass. An alternative procedure is to first fuse the chromite ore in theelectric furnace and then add the source of alumina, such as previouslyfused alumina of bauxite ore. The latter sequence of .steps is sometimesused to purify low grade chrome ore by reducing the iron outpreferentially, leaving behind'thechromium oxide, which reduces lessreadily.

For producing refractory blocks and. other pieces we, in general, castthe molten material into suitable molds by tapping or tilting thefurnace so that the molten mass flows into the mold. It is alwaysdesirable but particularly so in the latter case, that provision be madeto prevent molten material spilling into the water prevent too rapidloss of heat, by embedding them in a molding flask in which they aresurrounded by sand or heat insulating material.

The molds should be provided with risers of ample size to permitcomplete filling of the mold without interference by material freezingin the headers. If the riser is made wedge shaped with its minimumsection immediately adjoining the mold, removal of the excess materialconstituting a header is facilitated. After a mold is filled it is movedaway and additional molds also filled successively.

The molded pieces are left in the mold for heat treatment, or, in thecase of iron molds particularly, the pieces are taken from the moldsshortly after the outer walls of the casting have solidified and thenannealed without other than their own support. The headers should beremoved from the castings at this point by sledging, as the castings aretougher at this stage than when cold and there is less danger of theirbeing cracked by the hammering. With a header tapering to a smallersectional area next to the casting, removal in this manner is usuallysimple and fairly clean. 7

For. annealing we may utilize any of the customary annealing practicessuch as placing the molded pieces in a furnace and gradually reducingtheir temperature, covering the pieces with hot sand or other insulatingmaterial, or heavily insulating the pieces and allowing them to annealby virtue of their own heat or by that .of other blocks packed closelytogether with them.

The time-temperature curve required to secure given results naturallyvaries with the shape and size of the pieces and also with the exactcomposition of the material being cast. In general, it may be said thatif a given rate produces cracking it is obviously desirable to furtherretard the rate of cooling over some or all of the cooling range. Afterthe pieces are cold any objectionable remainder of the header or otherminor protuberances may be removed by chipping, or in minor cases bygrinding.

After the chromium-oxide-alumina mixture is melted, it may be allowed tocool in the furnace instead of pouring the molten refractory materialinto molds. In thus using the furnace itself as a mold, it is desirableto line it with a very light coating of refractory material so that themolten material may extend out to the edges to form a smooth block.Charging is carried on just as before, the electrodes being graduallywithdrawn and a block built up to the desired thickness. This method ofmolding has the disadvantage that only one mold can be filled at a time,but this is compensated for by the fact that practically no material islost in headers, etc., as in the other type of mold. It is sometimesdesirable to provide furnace molds of this type with a small dimensionaldraft to facilitate removal of the piece from the mold although due tothe considerable shrinkage after solidification this is in generalunnecessary.

When the fusion is cold, it is taken out as a pig and broken up orcrushed to fragments or particles of the desired size. The fragments orparticles of this material when reduced to the proper size are admirablysuited for use in making bonded refractory articles. Fragments of propersize are also suitable for use as abrasive grain. Bonded refractoriesmay be made in the usual wayby choosing proper grit mixes, addingsuitable bonding materials, forming by molding or other methods and thenfiring in a ceramic kiln to high temperatures. Preferably the bond usedshould, as a raw material, be similar in composition to the grainitself. I

Refractory shapes made in accordance with our invention are ofparticular value in various metallurgical applications, both in ferrousand non-ferrous furnaces, electric furnaces, chemical processes, etc.Their slag resistance makes them useful in general refractory service.

Fusions of chromium oxides and aluminum oxide made in accordance withour invention offer several advantages favoring their use as abrasives.Their crystals fracture in such a way as to make them effective asabradants. The fracture tends to be hackly instead of conchoidal as inordinary fused alumina. The material is tough, and is harder than purefused alumina.

While we have briefly referred to certain representative uses to whichthese fusions containing Cl'zOs and A120; may be put, others willreadily occur to persons skilled in the art to which they pertain. Wehave described certain details as to the elimination of at least a partof the impurities which are associated wtih ClzOs in the sources ofsupply of such oxides and preferred methods ofbalancing the amounts ofsuch impurities with respect to one another whereby the effect of one isnullified by the presence of the other so as to obtain a definitetrigonal form of crystallization. It is to be ,understood that ourinvention is not limited to details such as these, but may be otherwiseembodied and practiced within the scope of the following claims.

We claim:

1. A cast refractory article consisting principally of trigonal crystalsof a solid solution of alumina and chromic oxide, the chromic oxidebeing at least 8% and the alumina and chromic oxide together being atleast 80% of the weight of the article, said article also comprising 2to 6% FeO, 1 to 6% mo and 2 to 1% of acid oxides, the acid oxides beingpresent in amount sufficient to materially inhibit theformation ofspinel crystals within the article.

2. In the process of making a refractory article from a fushion ofchromite and a source of alumina whereby a solidified mass of trigonalcrystals of a solid solution of chromic acid and alumina results, saidarticle comprising magnesia in excess of 2% and ferrous oxide, the stepwhich comprises incorporating silica in the fusion in such proportion asto substantially decrease the formation of spinel crystals in the saidarticle.

3. A solidified fused mass formed from a fusion of chromite and a sourceof alumina, said mass.-

consisting principally of trigonal crystals of a solid solution ofchromic oxide and alumina, said two oxides constituting over by weightof the mass and the chromic oxide constituting at least 8%, said masscomprising also 1 to 7% msgnesia, 3 to 5% ferrous oxide and sufficientacid oxides to prevent the presence of more than 20% spinel crystals byvolume in the mass.

4. The method of making a composition of matter comprising in majorproportion a solid solution of aluminum oxide and chromium oxide, whichcomprises forming a fusion of materials comprising magnesia-containingchromite and a source of alumina whereby the said fusion contions notmore than 2.0% by weight of MgO and not more than 5% by weight ofironoxide and solidifying the fashion.

5. The method of making a composition of matter comprising in majorproportion a solid solution of aluminum oxide and chromium oxide, whichcomprises forming a fushion of magnesiacontaining chromite underreducing conditions to eliminate at least a portion of the oxides ofiron and silicon, diluting the said fushion with alumina whereby thesaid fusion contains not more than 20% by weight of MgO and not morethat 5% by weight of iron oxide, and solidifying the fushion.

6. The method of making a composition of matter comprising inmajorproportion a solid solution of aluminum oxide and chromium oxide, whichcomprises forming a fushion of materials comprising magnesia-containingchromite, chromic oxide, and a source of alumina whereby the saidfushion contains not more than 2.0% by weight of MgO and not more than5% by weight of iron oxide, and solidifying the fushion.

7. A cast refractory article solidified from a fushion consisting ofmagnesia-containing chromite and aluminum oxide, in which 'article themagnesia is maintained below 2.0% and the iron oxide below 5%.

8. A cast refractory article composed principally of trigonal crystalsof a solid solution of 8 to 30 parts chromic oxide and 50 to 85 partsalumina, said oxides constituting at least 80% of ill the article, saidarticle comprising also 3 to 6% FeO, 1 to 5% magnesia and sumcient acidoxides to substantially inihibit the presence of spinel crystals in thearticle.

9. A solidified fused mass consisting of alumina and chromic oxide, andup to 20% of other oxides including magnesia and acid oxides, said massbeing composed principally of trigonal crystals of a solid solution ofalumina and chromic oxide and being substantially free from spinelcrystals.

10. The method of making a composition of matter comprising in majorproportion a solid solution of alumina and chromic oxide in the form oftrigonal crystals. which comprises fusing together and thereaftercooling chromite and a source of alumina under such conditions that thecomposition shall contain not over 6% Eco, and

so proportioning the ingredients that the CH0:

shall constitute at least 8%, alumina plus CrsO: at least magnesia from1 to 6% and acid oxides a su mcient percentage to prevent the presenceof more than 20% of spinel crystals in the finished product. v

11. In the production of a product consisting predominantly of atrigonal crystalline mass of a solid solution of chromium oxide inaluminum oxide from a fusion of raw materials comprising enough magnesiato normally produce a substantial proportion of spinel crystals, thestep which comprises adding acidic oxides to the fusion in sumcientproportion to substantially inhibit the tendency of the magnesia tocrystallize in spinel form.

RAYMOND C. BENNER. HENRY N. BAUMANN, JR. GEORGE J. EASTER.

